Open FGD spray towers (i.e., those not having packings, trays or other means for facilitating gas-liquid contact) are simple in design and provide high reliability. They achieve intense gas-liquid contact and inherently entrain significant amounts of liquid in the effluent. Mist eliminators are employed to reduce the entrained liquid to an acceptable level. For a general discussion, see, for example, Rader and Bakke, in Incorporating Full-Scale Experience Into Advanced Limestone Wet FGD Designs, presented at the IGCI* Forum 91, Sep. 12, 1991, Washington, D.C. (*formerly the Industrial Gas Cleaning Institute, now the Institute of Clean Air Companies, Washington, D.C.).
As a practical matter, mist eliminators are designed to achieve a balance between droplet removal efficiency and pressure drop. Droplet removal efficiency improves with increases in initial droplet removal and decreases in reentrainment. Increases in fan power required to account for gas flow pressure drop across a mist eliminator can be a significant portion of the total system power usage.
Most mist eliminators for commercial FGD systems are of the impaction type--droplets of liquid are removed from a flowing gas stream by impacting with a surface. Larger droplets impact an obstruction placed in the line of flow because they tend to continue in a straight line regardless of changes in the direction of gas flow. Smaller droplets can follow the gas flow and avoid contact with obstructions. Impaction separators can take several forms, including rod banks, mesh pads, chevron and zig-zag baffles.
FIGS. 3-6 show various commercial impaction separators of the chevron type. They are formed of parallel, spaced vanes and can be used in both vertical and horizontal installations. They are all shown to have two passes, i.e., vane surfaces that change the direction of the gas flow. A stage is comprised of one or more adjacent passes. In the Figures, they are shown in one stage but are typically used in two stages and have been also used in one and three stage installations. Depending on many factors, stages can be comprised of one to four passes.
Total liquid removal efficiency is the difference between initial droplet removal and the amount of liquid reentrainment. The first should be maximized and the latter minimized. Among the factors favoring initial droplet removal are spacing between the individual impaction surfaces and the variation in the angle of these surfaces with respect to the direction of gas flow, the turn length for the zig-zag surfaces, the number of changes in gas flow direction (passes) and the number of stages. All of these factors, to the extent they can improve initial droplet elimination, affect pressure drop. A discussion of commercial mist eliminator design is provided by Jones, Mcintush, Lundeen, Rhudy, and Bowen, in Mist Elimination System Design and Specification for FGD Systems, presented Aug. 26, 1993, at the 1993 SO.sub.2 Control Symposium, Boston, Mass.
In some cases, gas straighteners are employed in the form of extended trailing edges of the vanes of a given stage. In some cases, these can improve multi-stage efficiency by adjusting the direction gas flow from one stage to the next. Also, hooks or channels on mist eliminator vanes have been employed to improve drainage of liquid collected. These features can reduce reentrainment by providing a protected drainage path for the liquid. These additional elements can, however, increase pressure drop and are difficult to keep clean in systems with suspended solids or precipitates.
Critical to pressure drop, initial droplet removal and reentrainment is the ability to clean the mist eliminator vanes while in continuous operation. Cleaning must be done in a manner effective to prevent encrustation, and with minimal entrainment of wash water. Commercial experience has been that increasing gas flow above certain velocities (higher for horizontal flow than for vertical) reentrainment can increase to unacceptable levels. The use of additional passes within a stage can reduce this tendency, but it makes cleaning the exit sides of the vanes very problematic because wash water should be applied to only the entrance side and not the exit. Introducing wash water at the exit side will unacceptably increase reentrainment as a portion of this liquid becomes entrained in the gas.
There is a need for a mist eliminator that will operate well at low superficial gas velocities, continue to function with low pressure drops and high removal efficiencies at high velocities, and be capable of effective cleaning.